1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to an improvement in a face plate and a shadow mask of a color cathode ray tube.
2. Description of the Related Art
FIG. 1 shows a shadow-mask type color cathode ray tube (color-CRT). The tube axis of color cathode ray tube 50 is defined as a Z axis. A major-axis direction perpendicular to the Z axis and passing through center 0 of panel 51 is defined as an X axis. A minor-axis direction perpendicular to the Z and X axes and passing through center 0 of panel 51 is defined as a Y axis. Color cathode ray tube 50 comprises substantially rectangular face plate 52, panel 51 having skirt 54 extending from a side edge portion of face plate 52, and funnel 56 coupled to panel 51. Funnel 56 has substantially cylindrical neck 58 housing an electron gun assembly. A phosphor screen is formed on the inner surface of face plate 52. A rectangular shadow mask is arranged on panel 51 to oppose the phosphor screen. The shadow mask is made of a thin metal plate, and has a large number of slit apertures. The shadow mask is arranged on the inner surface of face plate 52 to be separated at a predetermined distance therefrom. The periphery of the shadow mask is welded to a rectangular frame. Some elastically deformable supporting structures are welded to the frame. Since the supporting structures are engaged with panel pins mounted on panel 51, the shadow mask is supported on panel 51.
A plurality of electron beams emitted from the electron gun assembly housed in neck 58 are converged into the slit apertures of the shadow mask, and then land on the phosphor screen formed on panel 51. The phosphor screen is constituted by a plurality of stripe phosphor layers. The plurality of phosphor layers emit a plurality of colors upon landing of the electron beams. The shadow mask is arranged for causing electron beams to land on the predetermined phosphor layers.
In order to cause the plurality of electron beams to land on the predetermined phosphor layers, over 2/3 of the electrons of the plurality of electron beams emitted from the electron gun do not pass through the slit apertures, but are bombarded on the shadow mask and are converted to heat. Thus, the temperature of the shadow mask is increased, and the metal shadow mask is thermally expanded. Upon thermal expansion of the shadow mask, he relative position between the slit apertures of the shadow mask and the stripe phosphor layers of the phosphor screen is changed. A change in relative position between the slit apertures of the shadow mask and the stripe phosphor layers of the phosphor screen causes mislanding of the electron beams on the phosphor screen, thus degrading color purity of the color cathode ray tube. In order to correct the mislanding caused by the change in relative position between the shadow mask and the phosphor screen, supporting structures having a bimetal are employed. The supporting structures move the expanded shadow mask in a direction toward the phosphor screen upon movement of the bimetal, so that the distance between the shadow mask and the phosphor screen falls within an allowable range. Thus, the mislanding caused by the change in relative position between the shadow mask and the phosphor screen is corrected. However, when the phosphor screen is caused to emit light at high luminance and electron beams land to be concentrated on a portion of the phosphor screen within a short time interval, the shadow mask near the portion is strongly heated. The local heating of the shadow mask causes local mislanding of the electron beams. The local mislanding is a serious problem in the conventional color cathode ray tube.
U.S. Pat. Nos. 4,535,907 and 4,537,322 disclose an improvement in the panel of a cathode ray tube. U.S. Pat. No. 4,537,321 and Japanese Patent Disclosure (Kokai) No. 59-158056 (U.S. Pat. Serial No. 469,775) disclose a color cathode ray tube having a substantially flat face plate. In particular, since the face plate of the color cathode ray tube described in U.S. Pat. Serial No. 469,775 is substantially flat, mislanding of the electron beams is enhanced when the shadow mask is locally heated. The face plate of the color cathode ray tube, as shown in FIG. 2, has a large difference in distance between the central portion and an effective diameter end portion on the minor axis in the tube-axis direction, i.e., in the Z-axis direction, but has a very small difference in distance between an effective diameter end portion on the major axis and an effective diameter end portion on the diagonal line in the tube-axis direction, i.e., the Z-axis direction. In the panel, the face plate has a very large radius of curvature. Thus, since the peripheral portion of the face plate is substantially flat, the shadow mask also has an almost flat shape. Since the shadow mask is flatter from its central portion toward the peripheral portion, if a portion near the peripheral portion is heated by electron beam bombardment, the relative position between the phosphor screen and the shadow mask is changed, and the mislanding of electron beams is enhanced. As a result, the color purity of the color cathode ray tube is considerably degraded.
In the above problem, in order to examine a region of a color-CRT where local mislanding easily occurs, a signal generator for generating a rectangular window-shaped image pattern is used. The position and shape of the window-shaped pattern are changed to measure the mislanding of the electron beams. FIG. 3 shows beam pattern 5 by a large current for causing almost the entire surface of screen 6 to emit light at high luminance. In pattern 5 shown in FIG. 3, since the entire shadow mask is expanded, local mislanding relatively rarely occurs. FIG. 4 shows relatively elongated raster pattern 7 for causing a portion of screen 6 to emit light at high luminance. The largest mislanding occurs on the region where pattern 7 shown in FIG. 4 is located. The mislanding occurs for the following reasons. First, a CRT is designed such that an average anode current does not exceed a predetermined value. For this reason, a current intensity per unit area of the shadow mask in the pattern shown in FIG. 4 is higher than that in the large window-shaped pattern shown in FIG. 3. As a result, in the pattern shown in FIG. 4, the shadow mask is strongly heated and the temperature is increased rapidly. Second, mislanding most easily occurs at the position of raster pattern 7 shown in FIG. 4. In other words, the relative position between the slit apertures of the shadow mask and the corresponding stripe phosphor layers of the phosphor screen is easily changed at the position of the pattern shown in FIG. 4. This is because, since the electron beams obliquely pass through the slit apertures of the shadow mask, the position which electron beams land on the corresponding stripe phosphor layers of the phosphor screen is easily as well as largely changed by thermal expansion of the shadow mask. However, when the pattern is located near the central portion of the screen, if the shadow mask is thermally expanded due to heat, the direction in which the shadow mask is thermally expanded corresponds to the direction of the electron beams, and so the relative position between the slit apertures of the shadow mask and the corresponding stripe phosphor layers of the phosphor screen is not almost changed. When the pattern is located near the edge portion of the screen, since the shadow mask is fixed to the frame, thermal expansion can be prevented. Thus, mislanding most easily occurs on the region of the raster pattern shown in FIG. 4.
FIG. 5 shows a state of mislanding of electron beams shown in FIG. 4. Supporting structure 66 arranged on frame 63 which is welded to shadow mask 62 is engaged with stud pin 64 arranged on the inner surface of skirt 54 of panel 50. When electron beam 69 lands to cause phosphor screen 60 to emit light at low luminance, shadow mask 62 is not so heated, and is located at position A. In this case, electron beam 69 lands on the correct position of phosphor screen 60. When electron beam 69 lands to cause phosphor screen 60 to locally emit light at high luminance, shadow mask 62 is locally heated to a high temperature and is thermally expanded and shifted to position B. In this case, since slit aperture 63 of shadow mask 62 is moved near phosphor screen 60, the landing position of electron beam 69 on phosphor screen 60 is changed. As a result, the electron beam cannot land on the predetermined position of the phosphor screen.
A method of solving this problem is described in U.S. Pat. Nos. 4,677,339 and 4,697,119. In color cathode ray tubes described in the above patents, a radius of curvature in the Y-axis direction of a section obtained by cutting the shadow mask along a Y-Z parallel plane is changed. In the above patents, only the Y-axis direction of the color cathode ray tube is taken into consideration, whereas the X-axis direction is not taken in consideration.